1. Field of the Invention
This invention generally relates to a hand-held laser scanner for scanning and reading coded indicia, e.g. bar code symbols, with a first scan pattern and, more particularly, to a stand-alone fixture for selectively supporting the laser scanner thereon and, when so supported, for converting the first scan pattern to a different, second scan pattern operative for scanning and reading indicia located exteriorly of the fixture. Still more particularly, this invention relates to converting a hand-held scanner having a trigger, that initiates reading in a triggered mode when manually actuated, to a triggerless mode by simply mounting the scanner on the fixture, and also to supporting both triggered and triggerless hand-held scanners in the fixture, as well as to triggerless scanners in general. Furthermore, various digitizer circuits are disclosed for signal processing.
2. Description of Related Art
Hand-held laser scanners are well known. See, for example, U.S. Pat. Nos. 4,387,297; 4,409,470 and 4,806,742 as being representative of the art. A stand-alone laser scanning workstation, for example, see U.S. Pat. No. 4,369,361, having built-in laser, scanning and sensor components is also known. It is also known to provide a so-called "dummy" stand or fixture on which a hand-held laser scanner is optionally supported when not being held or used by a human operator. Such dummy stands typically serve mainly as convenient tabletop mounts or equipment perches to provide ready access of the scanner to the operator. In some cases, for example, see U.S. Pat. No. 5,105,070, the dummy stand is useful in changing the direction of a laser beam emitted by the supported scanner.
Each of the aforementioned hand-held scanners utilizes a trigger which, when manually actuated, initiates scanning and reading of the indicia. It has recently been proposed to provide a so-called "triggerless" hand-held laser scanner where, instead of a manually-operated trigger, the scanner employs an extra infrared light emitting diode (LED) and complementary infrared sensor, both mounted in a front end or nose of the scanner. When the nose is positioned close to a symbol, light from the LED reflects therefrom and is detected by the sensor. Additional control circuitry on-board the scanner turns on the laser and scanning components in response to such detection in order to initiate scanning and reading of the symbol.
The working range of this triggerless scanner is therefore limited by the sensing range of the infrared scanner. A symbol placed beyond the range of the sensor cannot be scanned because the sensor will not turn on the laser and scanning components. Moreover, aside from the additional expense of the infrared LED and sensor, this triggerless sensing technique can be inconvenient to use. To read another symbol, or to read the same symbol again, the scanner must be moved away from the symbol beyond the sensing range (e.g. about 15 cm) and then returned to be again positioned within sensing range. This can require large, tiring hand motions. In addition, the triggerless scanner is subject to many false reading attempts since any object positioned near the nose will turn the scanner on and, of course, this can include objects which bear no symbol.
It is also known in this art to construct a digital signal from an analog signal generated from a photoelectric transformation of light reflected off a symbol during scanning by using signal processing digitizing circuits which either use fixed threshold levels derived directly from the analog signal, or which compare a first derivative of the analog signal to a peak signal that follows that peaks of the analog signal. However, such known digitizing circuits are susceptible to errors due to such factors as high ambient light levels and noise, especially in cases where the reading beam spot is poorly focused, i.e., in long-range reading applications, or in reading very high density symbols.